Instruments for the measurement of infrared radiation are becoming increasingly important for a variety of commercial and non-commercial applications. Research into the development of uncooled sensors with response throughout the infrared spectrum has become particularly important due to the limitations on the operation of cooling systems. Uncooled infrared sensors would have important applications for space-based remote-sensing of thermal sources, night vision, target identification, thermal mapping, event detection, motion detection, and others. The limitations of the performance of the existing uncooled sensors often are the primary constraint to the performance of infrared imaging systems for many applications. As a result, there has been considerable investment in the development of uncooled infrared sensors.
A broad assortment of infrared detectors has been developed over the last 40 years. In most cases, they may be classified as either quantum or thermal detectors, depending upon whether the incoming radiation is converted to excitations which are collected, or is converted to heat and detected through changes in temperature. In general, a quantum detector which operates at detector temperature T.sub.d is usually superior to a thermal detector at the same temperature for infrared frequencies: hu&gt;&gt;k.sub.B T.sub.d, where h is Planck's constant and k.sub.B is Boltzmann's constant. However, for infrared frequencies: hu&lt;&lt;k.sub.B T.sub.d, thermal detectors represent the only functional technology. The operation of quantum detectors is limited by the availability of efficient photon conversion mechanisms, while the operation of thermal detectors is limited by the availability of sensitive thermometers. Only thermal infrared sensors operate in the mid-to-far infrared (.lambda.&gt;10 .mu.m) and at room temperature.
The pneumatic infrared detector, which was originally developed by Golay, is classified as a thermal detector. Golay's detector consisted of a small cavity filled with gas at room temperature. The cavity is separated from the surroundings by a window and a thin, flexible membrane. The membrane was coated on one side with a thin metallic film, which has significant absorption throughout the infrared spectrum whenever the sheet resistance of the film is approximately half of the impedance of free space. The trapped gas in the Golay cell was heated by contact with the membrane and expanded thermally, which forced the membrane to deflect outward. This deflection is usually detected with optical or capacitive displacement transducers. At present, these detectors are bulky, fragile, difficult to fabricate, and expensive. Nevertheless, they have been widely used, primarily because of their improvement in sensitivity over all other room-temperature detectors in the mid to far infrared range. Attempts to miniaturize the Golay cell for incorporation into focal plane arrays have been unsuccessful because of scaling laws which relate the sensitivity of conventional displacement transducers and their active area. The need for focal-plane arrays of uncooled detectors stimulated the development of pyroelectric detector arrays, the best of which are 5-10 times less sensitive than the Golay cell.
With the above considerations in mind, the present invention is based on the development of an improved Golay cell. This new sensor is constructed entirely from micromachined silicon components. To detect the motion of the membrane, the invention uses an electron tunneling displacement transducer. This sensor, like the assemblies used in Scanning Tunneling Microscopy (STM), detects electrons which tunnel through the classically forbidden barrier between a tip and a surface. As in the STM, the electron current is exponentially dependent on the separation between the tip and the surface. Through use of the electron tunneling transducer, the scaling laws which have prevented the miniaturization of the Golay cell are avoided.
Any new developments in transducer technology that avoid the constraints which relate to the sensitivity and dimensions of classical displacement transducers are very important. The STM, which was invented by G. Binnig and H. Rohrer of IBM Zurich and won the 1986 Nobel Prize in Physics, is based on the measurement of electron tunnel current between a surface of interest and a sharp tip, while the tip is raster-scanned across the surface. This device is capable of resolving atomic-scale structure on the surface of interest and bas enabled many pioneering discoveries of the structure and behavior of atoms at surfaces. The most important element of STM is the measurement of tunneling current between the tip and substrate. The tunneling current, I, has the following dependence on the separation, s, between a pair of metallic electrodes: ##EQU1## where .PHI. is the height of the tunneling barrier and V is the bias voltage; V is the small compared to .PHI., and .alpha.=1.025 (.ANG..sup.-1 eV.sup.-1/2). For typical values of .PHI.=5 eV and s=7.ANG., the current varies by an order of magnitude for each 1 .ANG. change in electrode separation. This sensitivity to relative position is superior to that available in all conventional compact transducers. Since tunneling only occurs in regions where the tip is within several .ANG. of the surface, the active area of the sensor is microscopic. The use of electron tunneling as the active element of a displacement transducer for generic sensor purposes has been pioneered at the Jet Propulsion Laboratory (JPL) over the last several years resulting in the construction of a series of proof-of-concept prototypes. These prototypes initially were constructed from a variety of materials, and served to illustrate that, if designed properly, tunneling could be used in a displacement transducer.